The present invention relates to an impedance measuring apparatus which permits high-speed, high-precision measurement of an impedance transfer function of a measuring object at low and even ultra-low frequencies.
FIG. 1 shows in block form a conventional synchronous detection type impedance measuring apparatus, in which a square-wave generator 11 provides 0.degree.-phase and 90.degree.-phase square waves at terminals 12 and 13, respectively. The 0.degree.-phase square wave at the terminal 12 is applied to a low-pass filter 14 to obtain a sine-wave output, which is amplified by an amplifier 15 and then applied to a measuring object 16. The current output I.sub.x of the measuring object 16 is converted by a current-voltage converter 17 into a voltage signal, which is provided to synchronous detectors 18 and 19. The voltage signal is multiplied by the square waves from the terminals 12 and 13 for synchronous detection. The outputs of the synchronous detectors 18 and 19 are integrated by integrators 21 and 22, respectively, to obtain integrated outputs R.sub.l and I.sub.m, which are selectively applied to an A-D converter 24 for conversion into digital form. Letting the resistance value of a feedback resistor 25 of the current-voltage converter 17 be represented by R, the A-D converter 24 yields a digital vector voltage R.multidot.I.sub.x =R.sub. l +jI.sub.m.
Provided that the low-pass filter 14 provides an output S.sub.il of a frequency f.sub.c as shown in FIG. 2A, the measuring object 16 and the current-voltage converter 17 produce distortions S.sub.i2, S.sub.i3, . . . as shown in FIG. 2A. On the other hand, the spectrum of a square wave contains relatively large odd higher harmonics as shown in FIG. 2B. Consequently, the above-mentioned distortions are synchronously detected by the higher harmonics in the synchronous detectors 18 and 19 and appear at their outputs. In short, the conventional measuring apparatus has a disadvantage that distortions by the measuring object 16 and the current-voltage converter 17 directly appear as measurement errors.
Another possible impedance measuring apparatus is such as depicted in FIG. 3. The output voltage of a sine-wave generator 26 is applied as an input voltage V.sub.in to the measuring object 16, and its current output I.sub.x is converted by the current-voltage converter 17 into a voltage signal, which is further converted by an A-D converter 27 into digital form. The digital signal is subjected to Fourier transformation by a Fourier transformer 28, obtaining an output S.sub.b corresponding to -R.multidot.I.sub.x. On the other hand, the input voltage V.sub.in from the sine-wave generator 26 is converted by an A-D converter 29 into a digital signal, which is Fourier-transformed by a Fourier transformer 31, obtaining an output S.sub.a corresponding to the input voltage V.sub.in. Thus, Z.sub.x =V.sub.in /I.sub.x =-R.multidot.S.sub.a /S.sub.b can be obtained.
This impedance measuring apparatus is free from the influence of the distortions by the measuring object 16 and the current-voltage converter 17, but since the A-D converters 27 and 29 convert waveforms into digital signals, they are required to be high-speed and high-precision, and hence are expensive. Further, difficulties are encountered in high-speed operation because of the digital Fourier transformation.
Where an error is contained in the transfer characteristic of the measuring system, a reference element is connected in place of the measuring object 16, the transfer characteristic of the measuring system is measured, and the measured results are used to perform a corrective operation on the output of the A-D converter 24 for the error in the transfer characteristic of the measuring system in the case of the measuring object 16 being connected.
It is customary to learn the physical state of a measuring object at the atomic level by measuring its temporal impedance variations while causing abrupt transient change in bias voltage, magnetic field, temperature etc. In such a case, a very quick transient impedance is measured, but a computing element which performs the above-mentioned corrective operation cannot respond to such a quick transient impedance. The prior art therefore employs a sampling system in which the transient response of the measuring object is repeated and the timing for measurement is shifted little by little accordingly. This sampling system has the defect of consuming much time for measurement.
In the case where a parasitic capacitance 30 is present in the measuring circuit as shown in FIG. 1, it is a general practice in the prior art to measure the output in the absence of the measuring object 16 and subtract the value of this output from a measured value obtained in the presence of the measuring object 16. With this system, however, the range of measurement of the impedance of the measuring object 16 and the accuracy of analysis are limited due to limitations on the dynamic ranges of the current-voltage converter 17, the synchronous detector 18, the integrator 21 and the A-D converter 24. That is, when current flowing through the measuring object 16 is small relative to a parasitic current flowing through the parasitic capacitance 30, the ratio of the parasitic current to the dynamic range of each of the current-voltage converter 17, the synchronous detector 18, the integrator 21 and the A-D converter 24 is large, reducing the accuracy of measurement.
It is therefore an object of the present invention to provide an impedance measuring apparatus in which a measurement error is free from the influence of distortions caused by a measuring object and a current-voltage converter.
Another object of the present invention is to provide an impedance measuring apparatus which is low-cost and capable of high-speed measurement.
Another object of the present invention is to provide an impedance measuring apparatus which permits measurement of a transient impedance in a short time.
Yet another object of the present invention is to provide an impedance measuring apparatus which is capable of high precision measurement without being affected by a parasitic impedance.